Such sleepers are located typically in the gravel bed and are usually installed at a distance of a ⅔ meter. The challenge in the structural implementation of parts that were previously partially produced with relatively simple means from wood, is that these must more and more be made of artificial substances, as it is no longer disputed that climate change affects timber and render it unsuitable for use as a mass application.
Newer designs have changed over to concrete that has to be pre-stressed through constructive means—for example, threaded steel bars must be molded into the concrete or set into the wet concrete and secured by means of nuts on the ends when the concrete hardens, such that the concrete tensile stability is maintained with the help of a fixed pre-tension. This is adjusted by the torque when tightening the nut. The downside is that such threaded bars either rust or if they are made of stainless steel, can tear or lose a part of the pre-stress over a period of time. The concrete can become brittle and crack, moisture and frost and the change in the concrete over time then do the rest, affecting the sustainability adversely. In concrete sleepers, cracks and hairline cracks are already planned during dimensioning, as these cannot be prevented in prestressed concrete—at least on the tension side. During load tests hairline cracks are admissible. Wooden sleepers do not have hairline cracks, but are affected by weather changes.